Evidence for Cosmic Inflation Theory Bites the (Space) Dust

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It is the announcement no one wanted to hear: The most exciting
astronomical discovery of 2014 has vanished. Two groups of
scientists announced today (Jan. 30) that a tantalizing signal —
which some scientists claimed was "smoking gun" evidence of
dramatic cosmic expansion just after the birth of the universe —
was actually caused by something much more mundane: interstellar
dust.

In the
cosmic inflation announcement, which was unveiled in March
2014, scientists with the BICEP2 experiment, claimed to have
found patterns in light left over from the Big Bang that
indicated that space had rapidly inflated at the beginning of the
universe, about 13.8 billion years ago. The discovery also
supposedly confirmed the existence of gravitational
waves, theoretical ripples in space-time.

But
in a statement today, scientists with the European Space
Agency said that data from the agency's Planck space observatory
has revealed that interstellar dust caused more than half of the
signal detected by the Antarctica-based BICEP2 experiment. The
Planck spacecraft observations were not yet available last March
when the BICEP2 science team made its announcement. [ Cosmic
Inflation and the Big Bang Explained (Infographic) ]

"Unfortunately, we have not been able to confirm that the signal
is an imprint of cosmic inflation," Jean-Loup Puget, principal
investigator of the HFI instrument on Planck at the Institut
d'Astrophysique Spatiale in Orsay, France,
said in the statement. The conclusion is the result of a
collaborative analysis by scientists with both BICEP2 and Planck,
using data from both telescopes as well as the Keck array at the
South Pole.

BICEP2, Planck and Keck all study the
cosmic microwave background (CMB), or light that is left over
from the Big Bang, and which can be seen in every direction in
the sky. One feature of the CMB that these experiments study is
its polarization, or the orientation of the light waves. (This
phenomenon may be familiar in the form of polarizing sunglasses,
which take advantage of light waves' orientation by blocking
light that becomes polarized as it bounces off the surface of
water.)

If inflation did occur when the universe was born, it would have
perturbed the fabric of the universe — which scientists call
space-time — creating what are known as gravitational waves,
according to the new statement. These waves would have then
created swirls in the polarization of the CMB, or what are called
B-modes. Thus,
the discovery of these B-modes would have meant both
confirmation of inflation and evidence of gravitational waves.

"Searching for this unique record of the very early universe is
as difficult as it is exciting, since this subtle signal is
hidden in the polarization of the CMB, which itself only
represents only a feeble few percent of the total light," Jan
Tauber, ESA's project scientist for Planck, said in the
statement.

When the BICEP2 team announced in March that it had detected
B-modes, the claim met
almost immediately with concerns about the influence of dust.
Interstellar dust gives off polarized light at similar
frequencies to the CMB. According to the ESA statement, the
BICEP2 team chose a region of the sky where the researchers
believed dust emissions would be low.

But the Planck results show that the light from dust is
significant over the entire sky — including the region where
BICEP2 purportedly observed B-modes. While BICEP2 only sees the
sky in one wavelength of light, Planck observes the universe in
nine wavelength channels, which help this instrument separate the
CMB signal from the background.

When the dust is accounted for, the signal identified by BICEP2
becomes too faint to be considered significant.

"The amount of gravitational waves can probably be no more than
about half the level claimed in our earlier study," said Clem
Pryke, a principal investigator of BICEP2 at University of
Minnesota.

The researchers say the joint collaboration does put an upper
limit on how many gravitational waves could have been created by
inflation, and shows that finding B-modes
will require more-sensitive measurements. The scientists
emphasized that the results do not eliminate the theory of
inflation. BICEP2, Planck, Keck and other CMB telescopes,
including
POLARBEAR, will continue to search for B-mode polarization
signals.

"The gravitational wave signal could still be there, and the
search is definitely on," said Brendan Crill, a leading member of
both the Planck and BICEP2 teams from NASA's Jet Propulsion
Laboratory in Pasadena, California.